1. Field of the Invention
The present invention relates to a multi-layer interference filter, in particular to a multi-layer interference filter for use in an optical communication and the like.
2. Description of the Prior Art
In general, a multi-layer interference filter shows a steep rise in spectrometric transmission versus wavelength if the number of layers thereof is increased, thereby improving the discriminating accuracy. However, on the other side, a disadvantage occurs in that the number of ripples ordinarily produced are increased as the number of layers are increased. In addition, it becomes more difficult to produce the filter as the number of layers are increased.
Accordingly, it is preferable that the production of a multi-layer interference filter is carried out while taking the above-described advantages and disadvantages into consideration.
A multi-layer interference filter has been used in optical communication and the like in recent years and is published in Shingaku-Giho OQE 82-14, page 70, left column. One of the filters having the construction of BK7//1.25M, 1.25L, (H/2 L H/2).sup.10, 1.25L, 1.25M//BK-7 has been described in a publication. Here, BK-7 is a trade name which designates the type of glass material composing the substrate, H designates TiO.sub.2 as a highly refractive substance layer, L designates SiO.sub.2 as a low refractive substance layer, and M designates Al.sub.2 O.sub.3.
However, a multi-layer interference filter of the above-described construction is composed of almost only quarter-wave optical thickness layers, so that it is advantageous in monitoring only three kinds of volatile substances and thus had distinct disadvantages. Besides, as shown by line B in FIG. 2, defects occur in that the rising characteristics are deteriorated and spectrometric characteristics are not always sufficient, for example the ripples are increased.